Prison Architect's hidden 3D mode was discovered the other week, and with the cat firmly out of the bag, developer Introversion said there may be updates on the way to the experimental mode. Said updates are now here as part of the grandly titled Update 5, which also adds hot water boilers, hot water pipes and, yes, hot water to the prison management sim. No wonder your prisoners have been trying to flee this whole time: they've had to make do with tepid showers.
Of the 3D view, Introversion says that it's "improved 3d mode with new shapes, new shaders operating on all objects, and better wall geometry", but that it "remains very experimental", so use at your own risk.
You'll need a boiler, hot water pipes, and a direct electrical connection to funnel hot water to your inmates, but it does come with a few benefits.
"When supplied with hot water, showers will also reduce a prisoner's Warmth need. When supplied only with cold water will apply a Frigid effect, which acts as a kind of short-term suppression while also spiking their Warmth and comfort needs."
There are other additions and fixes included in the update, detailed here, or if you're not in the mood for reading, you could watch this video: